For a demo recording like this, you're probably best off just adding midi drums yourself. If the problem is that you don't know how to write drums, it'll be useful to learn. Download some GP tabs, and see how the drum parts are written.

It's also next to impossible to add drums in after the fact unless the rest of the song has been perfectly recorded.

Well, midi drums with a (probably) unmastered, guitars-only demo could probably work.Oh, and TS, I know you're just trying to make it seem more reasonable, but this isn't "short". "Short" is less than three minutes. This is "average-length".